Field of the Invention
The present invention relates to a wire electrical discharge machine configured to quickly accurately perform an end face detection operation (see FIG. 1) for obtaining a reference machining position of an object of positioning/measurement (a workpiece or a fixture or the like to replace it) or measuring the geometries by relatively moving a wire electrode and the object of positioning/measurement to achieve contact and non-contact.
Description of the Related Art
In general, in a wire electrical discharge machine, a wire electrode and an object of positioning/measurement are relatively moved to contact each other, in order to ascertain their relative positions, and a relative position as a reference for machining is settled based on their “contact position” (relative position of the wire electrode at the time of contact). The “object of measurement” is supposed to be a workpiece or a detection fixture for positioning or the like to replace it. In the following description in this specification, the “object of measurement” is represented by a “workpiece” as required. Further, the relative position as the reference for machining is simply called a “reference position”, and this designation will also be used herein.
The “contact position” may be used directly as the reference position in some cases, or another relative position specifically related to the contact position may be used as the reference position in other cases. For example, the wire electrode may be passed through a round hole formed corresponding to a machining start region for the workpiece and relatively moved in three directions such that three positions of contact between the inner wall of the round hole and the wire electrode can be set and a center position of an circular arc that passes through the three points can be used as the reference position.
Thus, in order to accurately set the reference position based on the contact position between the wire electrode and the workpiece, the obtained contact position should naturally be accurate and reliable. In general, the contact position is detected in such a system that a detection voltage is applied between the wire electrode and the workpiece and the transition of the difference in the detection voltage corresponding to contact and/or non-contact is detected.
In settling the contact position between the wire electrode and the workpiece, a wire electrode 2 and a workpiece 1 are first kept in a definitely separate state (non-contact state), as shown in FIG. 1. The wire electrode and the workpiece in this non-contact state are relatively moved toward and brought into contact with each other with a voltage for contact detection applied between them (the workpiece 1 is moved toward the wire electrode 2 supported by a wire guide 3 as indicated by a movement 4 in FIG. 1). Conventionally, the relative position of the wire electrode at the point in time when the wire electrode and the workpiece are transferred from the non-contact state to a contact state is settled as the “contact position”. The “time point of the transfer from the non-contact state to the contact state” is determined based on a change of the detection voltage applied between the two elements involved in the transfer from the non-contact state to the contact state.
However, the change of the detection voltage achieved when the non-contact state is actually transferred to the contact state is unstable and low in reproducibility. This is believed to be mainly attributable to the occurrence of an unstable state that involves displacement caused by vibration of the wire electrode or the like as the wire electrode transfers from the non-contact state to the contact state. Such displacement naturally destabilizes the contact state of the wire electrode and the workpiece.
A technique disclosed in Japanese Patent Application Laid-Open No. 2004-66393 is an example of countermeasures against this vibration. First, a wire electrode is relatively moved toward a workpiece from a distant position so that they are fully in contact. When the wire electrode and the workpiece are in full contact, they are hardly displaced by the vibration of the wire electrode, so that the contact state is very stable. In order to ensure the attainment of the full-contact state, the start of the contact is detected in accordance with an appropriate criterion, and a further approaching movement toward the workpiece is then added for a predetermined distance, for example. Subsequently, release from the full-contact state is detected as the wire electrode and the workpiece are moved away from each other, and a relative position in which the release is caused is detected and set as a position representative of the border of the contact or non-contact between the wire electrode and the workpiece.
According to Japanese Patent Application Laid-Open No. 2000-107945, moreover, an approaching movement for a segment time is repeated and continued so long as a non-contact state is determined for each sampling period during the approaching movement by forward drive of a motor. If a contact state is determined during the approaching movement, the motor is stopped at the end of the segment time and reversely driven to start a separating movement. The separating movement for the segment time is repeated and continued so long as the contact state is determined for each sampling period during the separating movement. If the non-contact state is determined during the separating movement, the motor is stopped at the end of the segment time and a contact detection operation is performed in such a manner that the approaching movement is started by forward drive. Coordinate values of points in time when the approaching and separating movements are stopped are stored. An average of a large number of stored coordinate values is calculated and calculated average coordinate values are set as the coordinate values of the machining reference position. Thus, contact or non-contact detection positions are individually acquired to allow delay in detection errors due to the moving speed to be canceled out by averaging, so that the moving speed need not be extremely reduced.
In the case of the detection method disclosed in Japanese Patent Application Laid-Open No. 2004-66393, errors occur, though depending on the moving speed and the sampling period. If the speed of a movement (non-contact detection movement 15) of the workpiece 1 away from the wire electrode 2 is high, as shown in FIG. 2, errors in the non-contact detection position increase as indicated by a non-contact detection position (large error) 14. In order to reduce this error, the moving speed should be extremely reduced, as shown in FIG. 16. By reducing the speed of a non-contact detection movement 17, the errors in the non-contact detection position can be reduced as indicated by a non-contact detection position (small error) 16. While the errors can be reduced, the time for an end face detection operation is extended. Thus, in the case of the detection method disclosed in Japanese Patent Application Laid-Open No. 2004-66393, errors are liable to occur due to delay in detection or the like, so that the moving speed should be extremely reduced, and therefore, there is a problem that the time for the end face detection operation is extended. In the case of the detection method disclosed in Japanese Patent Application Laid-Open No. 2000-107945, moreover, an influence remains on the displacement caused by the vibration of the wire electrode or the like.
An ideal contact position between the wire electrode 2 and the workpiece 1 is such that a center 19 of the wire electrode 2 is located corresponding to the radius of the wire electrode as compared with an end face of the workpiece 1 (see FIG. 3). If the wire electrode 2 is displaced due to a factor such as its vibration or deflection or the difference (clearance) between the diameter of the wire electrode and the inner diameter of the wire guide, the center 19 of the wire electrode is located corresponding to the radius of the wire electrode plus a displacement amount as compared with the end face of the workpiece 1. Thus, there is a problem that an error (displacement amount) is caused by the vibration or deflection of the wire electrode or the difference between the diameter of the wire electrode and the inner diameter of the wire guide, so that an accurate reference position cannot be obtained.